It is well known in the art to reinforce ceramics and light metals such as Al, Mg, etc. (i.e., the matrix material) by dispersing a variety of reinforcement particles throughout the material. Common reinforcing particles are carbon/graphite, Al.sub.2 O.sub.3, glass, mica, SiC, wollastonite, alumino-silicate (e.g., Kao-wool), inter alia. Typically, the reinforcing particles will comprise about 3% by volume to about 30% by volume of the composite. The particles may be essentially equiaxed, or elongated (e.g., whiskers and fibers), and serve to improve the mechanical properties (e.g., strength, toughness, friction, fatigue resistance and wear resistance) of the composite over the properties of the metal or ceramic matrix material alone. Popular elongated particles (hereafter, fibrils) typically have an aspect ratio (i.e., length divided by diameter) of between about 3 to about 20. Their lengths vary from about 50 to about 200 microns and have diameters less than about 10 microns. Preferable lengths are between about 75 microns and 100 microns.
Reinforced composite materials are typically made by either one of two basic processes. In one process, reinforcements are simply mixed with the matrix material (e.g., molten metal) and together therewith cast as a slurry into an appropriate mold for shaping the finished product. In the other process, a self-supporting preform is made in the desired size and shape from the reinforcements, and the preform subsequently impregnated with the metal or ceramic matrix material by well known pressure or vapor infiltration techniques. In the latter process, it is particularly desirable that the preform be made to the actual size and configuration that it will be used in the finished molding/casting so that little or no subsequent machining or shaping thereof is required. According to this latter process, the preform is formed in an appropriate mold to the desired size and shape which may conform either (1) to that of the finished composite article, or (2) to only a particular defined portion of the finished composite article (e.g., a reinforced portion of an otherwise reinforcement-free article). Thereafter the preform is impregnated with the desired matrix material. The preform may be formed in a first mold and then transferred to a second mold where it is impregnated with the matrix material, or the preform may be formed and impregnated in the same mold.
A known technique for forming the preform comprises mixing the reinforcing particles uniformly throughout a fugitive binder (e.g., wax, polystrene, polyethylene, etc.), injecting the binder-particle mixture into a mold, removing (e.g., volatizing or dissolving) the binder, and finally bonding the residual particles together into a self-supporting structure. As is well known in the art, particle bonding may be achieved by sintering, or by providing the particles with a coating of colloidal SiO.sub.2 which, upon heating, acts like a high temperature glue for holding the particles together. Some of the disadvantages of the mix and mold technique are (1) the ofttimes inability to completely fill the mold cavity with a homogeneous mixture of the particles, (2) upper limits on the amount of particles that can be used while still being able to inject the mix, (3) the need to remove a large volume of binder (i.e., about 60% to about 85% by volume of the preform mixture) and (4) difficulty in avoiding planar, unreinforced areas which arise from flow lines and mating lines in mold.
All in all, the use of preforms is considered to be the preferred way to make composite materials. However, it has heretofore been difficult to uniformly and completely impregnate the preforms at commercially acceptable rates. It would be desirable to provide a self-supporting preform, which is readily impregnated without untoward sacrifice of the physical property(s) sought to be enhanced by the reinforcements.
It is an object of the present invention to provide a self-supporting, heterogeneous, reinforcement preform, wherein reinforcement particles are in the form of an open-cell reticulum, defining a plurality of reinforcement-free pores/cells which are readily fillable with matrix material. It is a further object of the present invention to form the aforesaid preform while foaming a fugitive binder therefor. It is a still further object of the present invention to provide a heterogeneous, reinforced material comprising a metal or ceramic matrix phase embedding a reinforcement phase which reinforcement phase comprises a three-dimensional reticulum comprising a plurality of randomly oriented thread-like portions interconnected one to the next via a plurality of nodes. These and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.
The present invention permits the making of readily impregnated metal matrix composites (hereafter MMCs) which have good wear-resistance and frictional properties, and readily impregnated ceramic matrix composites (CMCs) which are tougher than ceramics made without the reinforcements. The present invention provides a heterogeneous composite having a substantially consistent distribution of the reinforcements throughout the composite but with concentrations of the reinforcements at certain locations throughout and very little reinforcements elsewhere in the composite.